DataStore::sled_upsert returns a confusing error when upserting a decommissioned sled

[jgallagher]

The docs for sled_upsert note that attempting to upsert a decommissioned sled will fail, but the error it returns in such a case is confusing:

InternalError { internal_message: "Unknown diesel error creating Sled called \"d2de48dd-e9b2-4d5f-ac8c-d5ff0a97997d\": Record not found" }

I think the "Record not found" is coming because we filter on the state not being decommissioned, but we should map this to a more descriptive error than "unknown diesel error".